An image sensor may be a semiconductor device that converts an optical image into an electrical signal. Image sensors may be classified into categories, such as a charge coupled device (CCD) image sensor and a complementary metal oxide silicon (CMOS) image sensor (CIS).
A CIS may include a photo diode and a MOS transistor formed in a unit pixel. A CIS may obtain an image by sequentially detecting electrical signals of unit pixels in a switching manner. In a CIS structure, a photo diode region may convert a light signal to an electrical signal, and a transistor may process the electrical signal. A CMOS image sensor may include a photodiode and a MOS transistor within a unit pixel to sequentially detect electrical signals of each unit pixel, implementing an image. A photo diode region and a transistor may be horizontally arranged in a semiconductor substrate.
In a horizontal type CIS according to the related art, a photo diode and a transistor may be horizontally formed adjacent to each other on and/or over a substrate. Therefore, an additional region for forming a photo diode may be required.
Also, in a related art horizontal type CIS, it may be difficult to achieve an optimized process of concurrently forming a photodiode and a transistor. That is, a shallow junction may be required to meet low sheet resistance in a rapid transistor process, but such a shallow junction may not be suitable for a photodiode.
Further, in a related art horizontal type CIS, extra on-chip functions may be added to an image sensor. Thus, a unit pixel size may need to be increased to maintain a sensitivity of an image sensor or an area for a photodiode may need to be decreased to maintain a pixel size. However, if a pixel size increases, a resolution of an image sensor may decreases. If an area of a photodiode decreases, a sensitivity of an image sensor may become poor.